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Example research essay topic: Constructed Wooden Molds Fines Mixture Pour Mixtures Concrete - 2,443 words

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... ysi cal, thermal, and sometimes the chemical properties influence the performance of concrete. Aggregate is cheaper than cement and it is, therefore economical to put into the mix as much of the former and as little of the latter as possible. But economy is not the only reason for using aggregate: it confers considerable technical advantages on concrete, which has a higher volume stability and better durability than hydrated cement paste alone. General classification of aggregates: The size of aggregate used in concrete ranges from tens of millimeters down to particles less than one-tenth of a millimeter in cross-section. The maximum size actually used varies but, in any mix, particles of different sizes are incorporated, the particle size distribution being referred to as grading.

In making low-grade concrete, aggregate from deposits containing a whole range of sizes, from the largest to the smallest, is sometimes used; this is referred to as all-in or pit-run aggregate. The alternative, always used in manufacture of good quality concrete, is to obtain the aggregate in at least two size groups, the main division being between fine aggregate, often called sand not larger than 5 mm or 3 / 16 in. , and coarse aggregate, which comprises material at least 5 mm or 3 / 16 in. in size. The design of concrete mixes has been made much more complex by the availability of many different cementitious materials as well as admixtures to reduce water requirement, entrain air, accelerate or retard setting, and reduce permeability or shrinkage. It may help to start with the old fashioned idea that the concrete consists of cement, coarse aggregate, fine aggregate and water. Historically the problem of mix design has been seen as to select suitable aggregates and determine their optimum relative proportions and the cement requirement to produce a given strength at a given slump.

Early investigators tended to be concerned with how to define and produce ideal concrete. Frequently this meant trying to determine the ideal combined grading of the coarse and fine aggregate and therefore how these materials should be specified and in what proportions they should be combined. Today, the consideration of concrete mix should be: &# 61607; Use available aggregates rather than searching for ideal aggregates. &# 61607; Recognize that there is no concrete ideal for all purposes, but rather define what is required for particular purpose. &# 61607; Understand that there will be competition based on price While strength is often not the most important requirement, the reason for its use as a criterion is shown by the step following its selection in most mix design procedures. This is to convert the strength requirement into a water / cement (w / c ) ratio.

The relationship between strength and w / c ratio is generally attributed to Abram's in the United States. (Attachment 2) As a measure of the quality of concrete the compressive strength has become one of the most important standards, all strengths of concrete- compression, tension, shear and flexure - being related. The compressive strength is usually employed as a relative measure of strength under other loading, such as shear or tension. In nearly all-structural applications, the concrete is required to resist compressive stresses. However, some cases in which the concrete carries a load other than compressive. For example, a beam is in flexure and will break in the middle because the bottom fibers are in tension and concrete is weak in tension. We overcome this weakness by inserting reinforcing steel in the bottom of the beam to provide resistance to flexural loading or bending, the steel having a high tensile strength.

In concrete beam loaded in flexure, or bending, the bottom fibers are in tension. Reinforcing steel placed in the bottom of the beam gives it tensile strength to enable it to support such a load. The commonly accepted strength test is made when the concrete is 28 days old, because, concrete reaches its final strength on the 28 th day. (Attachment 6) A durable concrete is one that will withstand, to a satisfactory degree, the effects of service conditions to which it will be subjected, such as weathering, chemical action, and wear. Numerous laboratory tests have been devised for measurement of durability of concrete, but it is extremely difficult to obtain a direct correlation between service records and laboratory findings. (Attachment 2) a) Weathering Resistance: Disintegration by weathering is caused mainly by the disruptive action of freezing and thawing and by expansion and contraction, under restraint, resulting from temperature variations and alternate wetting and drying. Concrete can be made that will have excellent resistance to the effects of such exposures if careful attention is given to the selection of materials and to all other phases of job control.

The purposeful entertainment of small bubbles of air also helps to improve concrete durability by decreasing the water content and improving place ability characteristics. b) Resistance to Chemical Deterioration: Concrete deterioration, attributable in whole or in part to chemical reactions between alkalies in cement and mineral constituents of concrete aggregates, is characterized by the following observable conditions &# 61607; Cracking, usually of random pattern on a fairly large scale (Pictures 1, 2) &# 61607; Excessive internal and overall expansion &# 61607; Cracks that may be very large at the concrete surfaces (openings up to 1. 5 in. have been observed) but which extend into the concrete only a distance from 6 to 18 inches c) Resistance to Erosion: The principal causes of erosion of concrete surfaces are; cavitation, movement of abrasive material by flowing water, abrasion and impact of traffic, wind blasting, and impact of floating ice. (Pictures 3, 4) Permeability is the ease with which liquids or gases can travel through concrete. There are three avenues by which water can penetrate concrete: &# 61607; Gross voids arising from incomplete compaction, often resulting from segregation. &# 61607; Micro (or macro) cracks resulting from drying shrinkage, thermal stresses or bleeding settlement. &# 61607; Pores or capillaries resulting from mixing water in excess of that which can combine with the cement. Gross voids may be regarded as too obvious a cause to be included. However, they are worth mentioning because they may be made more likely by action which would otherwise reduce porosity, i.

e. harsh, low slump mix will have a low water content and a richer mortar (higher cement / sand ratio) than a sandier mix of equal strength. Obviously a low permeability concrete must be such that it will be fully compacted by the means available. It must not depend on unrealistic expectations of workmanship. A concrete that can be readily compacted is said to be workable, but to say merely that workability determines the ease of placement and the resistance to segregation is too loose a description of this vital property of concrete. The desired workability in any particular case would depend on the means of compaction; likewise, workability suitable for mass concrete is not necessarily sufficient for thin, inaccessible, or heavily reinforced sections.

For these reasons, workability should be defined as a physical property of concrete alone without reference to the circumstances of a particular type of construction. This is a test used extensively in site all over the world. The slump test does not measure the workability of concrete, although it is described as a measure of consistency, but the test is very useful in detecting variations in the uniformity of a mix of given nominal proportions. The mould for the slump test is a frustum of a cone, 300 mm (12 in. ) high. It is placed on a smooth surface with the smaller opening at the top, and filled with the smaller opening at the top, and filled with concrete in three layers. Each layer is tamped 25 times with a standard 16 -mm (5 / 8 in. ) diameter steel rod, rounded at the end, and the top surface is struck off by means of a sawing and rolling motion of the tamping rod.

The mould must be firmly held against its base during the entire operation; this is facilitated by handles or foot-rests brazed to the mould. Immediately after filling, the cone is slowly lifted, and the unsupported concrete will now slump- hence the name of the test. The decrease in the height of the slumped concrete is called slump, and is measured to the nearest 5 mm (1 / 4 in. ). In order to reduce the influence on slump of the variation in the surface friction, the inside of the mould and its base should be moistened at the beginning of every test, and prior to lifting of the mould the area immediately around the base of the cone should be cleaned of concrete which may have dropped accidentally. (Attachment 4) For the concrete mix: Scup, bucket, shovel, and pre-constructed wooden mould (6 " x 6 " x 3 '). (The steel make concrete mould on the right side is similar to our wooden mold, which is also open top) For the slump test: Slump cone, inspection scale, base plate, scoop, trowel, brush, and tamping rod. For testing: hydraulic concrete beam test machine. (This test machine shown is similar to the one we used, shown here for visualization purpose only) 3 / 8 " diameter by 3 ' Rebar 1 / 2 ' diameter by 3 ' Rebar 2 pieces Concrete Recipe: Make 2 blocks of concrete (sample 1 and 2) with the following proportions. 90 lbs of aggregates and fines mixture, Pour mixtures into two pre-constructed wooden molds. 2 rebar's are pre-installed for each concrete sample located parallel at 2 inch up from the bottom of the concrete and 2 inches apart.

Make 1 block of concrete (sample # 3) with the following proportions. 45 lbs of aggregates and fines mixture, Pour mixtures into two pre-constructed wooden molds. 2 rebar's are pre-installed for each concrete sample located parallel at 2 inch up from the bottom of the concrete and 2 inches apart. Curing period for all 3 concrete beams is 15 days. Curing samples are located at the green house open area. 1. Pour water and cement and mix it in the bucket using the shovel. Then add aggregates and fines mixtures and continue stir until all materials are equally mixed. 2. Perform slump test using the slump test equipments and allow the testing concrete slump to have 4 -inch separation between the top of the slump and the inspection scale. (if the slump test is failed, then add more aggregates and fines or add more water until the slump test is 4 -inches apart) 3.

Install the rebar's to all three pre-constructed wooden molds. (Do this step prior to pour concrete into the molds) 4. Pour concrete into the first 2 molds. 5. Repeat step 1 through 4 for the last concrete sample with the indicated concrete mix proportion. 6. Place the samples in front of the Green House for cure. 7. Take the cured concrete beams out of the wooden molds and test the flexural strengths using the hydraulic concrete-beam machine. 8. Relate our results to the real situation in the formal report.

Water/Cement/Aggregates RatiFlexural Strength (psi) 5, 980 10, 040 5, 260 Since there was enough time for our experiment to cure the samples for 28 days, our samples were only cure for 14 days. As a result of this time period the result we got for the strength are 86 % efficient since the concrete has full hardness after 28 days of curing. (Attachment 6) Different results were obtained for each sample since water / cement /aggregate ratios and rebar sizes differed. Sample 1 and sample 2 had the same water / cement /aggregate ratio, which is 1: 2: 3 but they had different rebar sizes. Sample 1 had 3 / 8 -inch diameter reinforcement where sample 2 had -inch diameter reinforcement.

The compression failure for sample 1 occurred at 5, 980 lbs where sample 2 failed at 10, 040 lbs. The strength obtained from sample 2 was much higher than sample 1 since reinforcement keeps the concrete together and makes it harder to crack. This shows that the reinforcement size makes the strength of the concrete much higher. Sample 1 and sample 3 have same sizes of reinforcement bars (3 / 8 -inch diameter) but their water / cement /aggregate ratio is different.

Sample 1 has 1: 2: 3 w / c /a ratio and sample 3 has 3: 1: 9 w / c /a ratio. As a result of different water / cement ratio, where sample 1 has a less water / cement ratio, it has more strength. The strength for sample 1 is 5, 980 lbs where sample 3 has strength of 5, 260 lbs. As a result of our experiment, the two important aspects in concrete design (durability and strength) are water / cement ratio and the size of the reinforcement bar that are used.

It is seen that increasing the size of the reinforcement increases the strength of the concrete. That's the reason why heavy reinforcement is used and building foundations and columns whereas small sizes of reinforcement are used in slab sections. In a building the most compressive forces are on the foundation and this is the reason why larger size of reinforcement bars are used. Comparing our sample 2 with the other samples, it can be seen that sample 2 is much more suitable to use in the foundation than the other 2 samples since it has almost twice the strength. The results obtained from the experiment agree with the primary objective. It is seen that we cannot just mix cement, water, and aggregate in a random ratio and build a structure.

There are many different mixes of concrete that serve different purposes. It is a wise idea to test the concrete when building a large structure so that unexpected failure doesn't happen. Studying and getting ready to become civil engineers, we learned a lot of information by doing the concrete research and also experienced this information practically during our experiment. This meets our secondary objective. Bibliography: References: Day, K. (1999). Concrete Mix Design, Quality Control and Specification, 2 nd edition; E & FN Spon.

Neville, A. (1996). Properties of Concrete, 4 th Edition; John Wiley & Sons, Inc. Neville A. , Brooks J. (1987). Concrete Technology; Longman Scientific & Technical. Shah S. , Ahmad S. (1994).

High Performance Concrete: Properties and Applications; Mc Great-Hill, Inc. U. S. Department of the Interior. (1981). Concrete Manual: A water resources technical publication, 8 th edition; United States government printing office. Waddell, J. (1978).

Fundamentals of Quality Precast Concrete; National Precast Concrete Association.


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Research essay sample on Constructed Wooden Molds Fines Mixture Pour Mixtures Concrete

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